header & video fixes for unplugged levels

These were changes I had planned to make after phase 1:

Fixing the header for Stages with only one puzzle (which is true for all unplugged activities). It will just say "Stage 1" and not include the Puzzle nav.

Fixing autoplay videos for Unplugged activities.  Two problems here: (1) some styles were in blockly's css but needed to be in the dashboard. We can remove them from blockly next. (2) The #notes-content block was inside the blockly partial but since it is now referenced on non-blockly levels (unplugged activities), it needs to be pulled up a level into levels/show.html.haml.

I also removed a style for prizes that didn't need to be added yet.  It will come back in a future PR.

And I fixed a bug on sizing the autoplay video dialog.  The border was being clipped off the bottom because the IFRAME was being sized with too much height.  Borders were not being considered when computing the desired height.